As shown in Japanese Patent Publication No. 2003-311667, an electric motor such as a servomotor is conventionally used as an actuator for moving a joint of a robot. It is because motors are relatively readily available. However, the use of a motor is likely to make the robot larger and heavier. A fluid cylinder such as an air cylinder is lighter and more compact in size than a motor. In addition, the fluid cylinder is simple in structure and maintenance thereof is easy. The motor having such advantages is useful as an actuator for a robot.